1. Field of the Invention
The present invention relates to a display device including a current drive type electro-optic element (referred to as a display element or a light emitting element as well).
2. Description of the Related Art
There is known a display device using a current drive type electro-optic element a luminance of which changes in accordance with a current caused to flow through the current drive type electro-optic element as a display element for a pixel. For example, the current drive type electro-optic element is typified by an Organic Electro-Luminescence (EL) element such as an Organic Light Emitting Diode (OLED). An organic EL display device using an organic EL element is a so-called self emission type display device using an electro-optic element, as a self light emitting element, the display element for the pixel.
The organic EL element is the electro-optic element which includes an organic thin film (organic layer) obtained by laminating an organic hole transporting layer and an organic light emitting layer between a lower electrode and an upper electrode, and which utilizes a phenomenon that when application of an electric field across the organic thin film results in light emission. In this case, a chromogenic gradation is obtained by controlling a value of a current caused to flow the organic EL element.
The organic EL element consumes a less power because it can be driven with a relatively low applied voltage (for example, 10 V or less). In addition, the organic EL element is a self light emitting element which emits a light for itself. Thus, weight saving and thinning are readily carried out for the organic EL element because the organic EL element does not desire a subsidiary radiating member such as a backlight necessary for a liquid crystal display device. In addition thereto, since a response speed of the organic EL element is very high (for example, on the order of several microseconds), no residual image in a phase of display of a moving image occurs. From these advantages described above, in recent years, a planar self light emission type display device using the organic EL element as the electro-optic element has been actively developed.
In an organic EL display device using the current drive type element such as the organic EL element as the electro-optic element, a drive signal (voltage signal) corresponding to an input image signal written to a hold capacitor is converted into a current signal by a drive transistor, and the resulting drive current is supplied to the organic EL element or the like. With such a drive system, when a drive current value differs, an emission luminance differs so as to follow the drive current value. For this reason, in order to cause the organic EL element to emit a light at a stable luminance, it is important to supply a stable drive current to the electro-optic element. Therefore, in general, a constant current drive system is adopted. However, threshold voltages and mobilities of the drive transistors for driving the electro-optic elements disperse due to a change in process. In addition, the characteristics of the electro-optic element such as the organic EL element change with time. When there are the dispersion of the characteristics of the drive transistors, and the change in characteristics of the electro-optic element, even in the case of the constant drive system, an influence is exerted on the emission luminance.
For this reason, for the purpose of uniformly controlling the emission luminance over the entire picture on the display device, a mechanism for correcting a change in luminance due to the change in characteristics of the drive transistor or electro-optic element described above within the pixel circuits has been variously examined. Such a mechanism, for example, is described in Japanese Patent Laid-Open No. 2007-310311.
For example, with the mechanism described in Japanese Patent Laid-Open No. 2007-310311, there are proposed a threshold correcting function and a mobility correcting function of causing a drive current to be constant when there are the dispersion and the temporal change in threshold voltage, and mobilities of the drive transistors. Also, there is proposed a bootstrap function of causing the drive current to be constant even when there is a temporal change in current-voltage characteristics of an organic EL element.